1. Field of the Invention
This invention is a process for preparing aldehydes having 3-26 carbon atoms by reacting olefins having 2-25 carbon atoms with hydrogen and carbon monoxide in the presence of a catalyst in a tube reactor.
2. Discussion of the Background
Aldehydes are used in the synthesis of many organic compounds, for example, alcohols and carboxylic acids. In addition, aldol condensation of aldehydes and subsequent hydrogenation of the condensate provides alcohols having twice the number of carbon atoms as the starting aldehydes. Alcohols prepared by the hydrogenation of aldehydes are used, for example, as solvents and as intermediates for the production of plasticizers and detergents.
It is known that aldehydes and alcohols can be prepared by reaction of olefins with carbon monoxide and hydrogen (i.e., hydroformylation, oxo process). The reaction is catalyzed by hydridometal carbonyls, preferably those containing metals of Group VIII of the Periodic Table. Apart from cobalt, which is widely used industrially as catalyst metal, rhodium has acquired increasing importance in recent times. In contrast to cobalt, rhodium allows the hydroformylation reaction to be carried out at low pressures, and the hydrogenation of the olefin starting materials to form saturated hydrocarbons takes place to a significantly lesser extent when using rhodium catalysts than when using cobalt catalysts.
In the industrial hydroformylation process, the rhodium catalyst is formed in situ from a catalyst precursor, synthesis gas and possibly other ligands. When using modified catalysts, the modifying ligands can be present in excess in the reaction mixture. Ligands which have been found to be particularly useful are tertiary phosphines or phosphites, which allow the reduction of the reaction pressure to values of significantly less than 300 bar.
However, separating off the reaction products and recovering the catalysts homogeneously dissolved in the reaction product are problems in this process. In general, the reaction product is distilled from the reaction mixture. However, because of the thermal sensitivity of the catalyst or the products formed, in practice, this route is possible only for the hydroformylation of lower olefins having 5 or less carbon atoms in the molecule.
Industrially, C4- and C5-aldehydes are prepared by hydroformylation, for example as described in DE 32 34 701 or DE 27 15 685.
In the process described in DE 27 15 685, the catalyst is dissolved in an organic phase comprising the product and high boilers formed from the product. Olefin and synthesis gas are added into this mixture. The product is carried from the reactor in the gas phase with the synthesis gas, or is taken off as a liquid. Since the activity of the catalyst slowly decreases over time, some of the catalyst has to be continually bled out of the reaction, together with high boilers, and replaced by an equivalent amount of fresh catalyst. Recovery of the rhodium from this bleed stream is absolutely necessary due to its high cost. A disadvantage of this process is the complexity of the isolation and purification of the product and recovery of the rhodium.
The process of DE 32 34 701 overcomes this disadvantage by dissolving the catalyst in water. The rhodium catalyst is solubilized in water by using trisulphonated triarylphosphine ligands. Olefin and synthesis gas are added to the aqueous catalyst phase, and the product produced by the reaction forms a second immiscible organic liquid phase. The liquid phases are separated from one another outside the reactor and the catalyst phase which has been separated off is returned to the reactor. However, this process provides lower space-time yields than processes in which the catalyst is dissolved in a liquid organic phase, because the catalyst is primarily soluble in the aqueous phase, whereas the olefins are primarily soluble in the organic phase, but are virtually insoluble in the aqueous phase. The already low solubility of the olefins in the aqueous phase decreases further with increasing molar mass of the olefins. As a result, higher aldehydes cannot be prepared economically by this process.
Addition of an organic solvent which is soluble in the aqueous catalyst phase increases the rate of the hydroformylation reaction. The use of alcohols such as methanol, ethanol or isopropanol as cosolvents increases the reaction rate, but has the disadvantage that the rhodium catalyst is then present in the product phase (B. Cornils, W. A. Herrmann, Aqueous-Phase Organometallic Catalysis, Wiley-VCH, p. 316-317) and is thus removed from the reactor. An increase in the reaction rate can also be achieved, for example, by addition of ethylene glycol. However, adding ethylene glycol reduces the selectivity of aldehyde formation since ethylene glycol can form acetal derivatives with the aldehyde (V. S. R. Nair, B. M. Bhanage, R. M. Deshpande, R. V. Chaudhari, Recent Advances in Basic and Applied Aspects of Industrial Catalysis, Studies in Surface Science and Catalysis, Vol. 113, 529-539, 1998 Elevier Science B.V.).
EP 0 157 316 describes adding solubilizers such as carboxylic acid salts, alkyl polyethylene glycols or quaternary onium compounds to increase the reaction rate in the hydroformylation of 1-hexene. Depending on the solubilizer used, the productivity was increased by a factor of 4. Increasing the reaction rate by addition of polyglycols (e.g. PEG 400) and polyglycol ethers is also known. Thus, DE 197 00 805 C1 describes the hydroformylation of propene, 1-butene and 1-pentene, and DE 197 00 804 C1 describes the hydroformylation of higher olefins such as 1-hexene, 4-vinylcyclohexene, 1-octene, 1-decene or 1-dodecene. In both of these processes, the use of solubilizers does increase the reaction rate, but the separation of the aqueous catalyst phase and organic product phase is more difficult, which results in losses of catalyst due to the increased solubility of the catalyst in the organic phase, as well as losses of desired products which become soluble in the aqueous phase. If the amount of solubilizer is reduced to minimize these losses, the reaction rate is simultaneously reduced again.
DE 199 25 384 states that the space-time yield of aldehydes in the hydroformylation of olefins in a multiphase reaction, in which one of the phases is a continuous catalyst phase, can be improved if the reaction is carried out in a flow reactor at a loading factor B greater than 0.8, rather than in a stirred reactor. This process for hydroformylating olefins by means of a multiphase reaction employs very high loading factors in the tube reactor, i.e., extremely high mixing of the phases. Phase transfer reagents, surface-active or amphiphilic reagents or surfactants can be added to the catalyst phase, and water is the preferred solvent for the catalyst.
It is therefore an object of the present invention to provide a process for hydroformylating olefins which provides high space-time yields and selectivities.
Surprisingly, it has been found that olefins may be the hydroformylated in a multiphase reaction, at high yields and with low formation of by-products, if the catalyst phase comprises a solvent mixture. The present invention accordingly provides a process for hydroformylating one or more olefins having from 2 to 25 carbon atoms by means of a multiphase reaction in a tube reactor, wherein
a) the catalyst is present in the continuous phase,
b) the continuous phase contains a solvent mixture,
c) at least one olefin is present in the disperse phase, and
d) the loading factor of the tube reactor is equal to or greater than 0.8.
In the present invention, the hydroformylation is carried out in a tube reactor, i.e. a flow tube. The catalyst phase and the dispersed phase containing at least one olefin are pumped into the tube reactor. After the reaction, the reaction mixture is separated into a product phase and a catalyst phase, and the catalyst phase is recirculated to the tube reactor. The product phase is removed, and the aldehydes can be isolated and purified by distillation. Aldehydes prepared by this process can be hydrogenated to prepare alcohols, used in aldol condensations, or oxidized to prepare carboxylic acids.